An experimental investigation was carried out to study the natural convective air cooling of the printed-circuit-boards (PCBs) array which was set up vertically in a closed casing. The results show that there exists an unstable thermal boundary layer on the top wall, which brings about vortex flow and temperature oscillation in the casing. Because of the thermal instability the air circulation in the casing is unsteady. Time-varying recirculation presents in the PCBs array. Experimental results were correlated in the form of Nu = f(Ra,r) with Rayleigh-number ranging from 102 to 105 at Pr = 0.71.